We can give you on it... sure. But the bottom line is: It isnt possible, IF you are trying to build the line in to your family.

If you simply go in the VIEWPORT type, and turn on Extension Line, its possible. But then you cant right justify things with the View Title Line.

The on why its not possible:

When you create a Family (Annotation, View Title Family), it has both Types and Instances.

But when you place a Viewport, you arent using "instances" of the family, you are placing "instances" of a VIEWPORT TITLE, which has the Annotation TYPE selected in its Type Properties. Ergo: There is no place to change the instance parameters of the Annotation. If the Viewport was a Family, you would link the parameter of the Nested Annotation Family, in to the Viewport Family. But Viewports arent accessible in that way, so you cannot do it.

So your choices are:

1. Use the OOTB Extension Line for Viewports
2. Dont have a line that extends a variable amount
3. Use Types, and have a bunch of types, even though then people who think they know better will try to passive aggressively mock you on the forums.

okay; thanks for the reply! i understand. do you have a link or anything that explains it? does it have to do with being associated with a view?

@deepblue02101996
That's just the way it is, like trying to explain why do we have System Families, & why can't they be Loadable? It just works better that way.

What I did that could help you, was Create Types that represent the Column Cell count; like my current firm has a sheet grid standard of 5 Rows & 6 Columns. So I based my Types (Line Length in View Title) based on how many Column Cells the View is occupying. Then in the Project create my 6 Viewport Types based on Cell counts. Then I have types for No Title, Standard ("Default Viewport"; accounts for Legend Column, or 5 Cell width, for Typical Plans unless a bottom Legend space, which the 6 Cell will be used), Title Only, & the ootb Adjustable Extension Line.
See Example....
So instead of having to always draw them in as they were before my arrival. Now they can swap Viewports & snap align once a View Title is in place to a Row/Column. It's faster than drawing lines, & then if a Sheet needs to be reorganized to squeeze in another View within the Sheet Grid Layout, then easier instead of redrawing lines. Not my cup of tea, but I'm just allowing the office graphic standards to be more efficient than drawing lines.
Sheet Grid.PNG

nice! thats a nice work flow! ill have to check it out